Hydrodynamic foil face seal

ABSTRACT

A foil thrust bearing forms the primary rotating interface for a film riding face seal. This face seal includes a spring system that will allow the entire assembly to translate axially relative to a static attachment. The complete assembly will form a hybrid foil/film riding face seal that shows much promise at being sufficiently flexible to enable operation in a gas turbine engine. The seal includes a flexible top plate having a non-flat surface under its working conditions, and a spring support system to allow the top plate to accommodate axial excursions and out-of-flat distortions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/303,588, filed Jul. 6, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the field of gas riding face seals.

Gas turbine engines bleed off some of the compressed air from theprimary gas path into so-called secondary flow circuits for variousreasons, mainly to cool various components within the engine. Thiswithdrawal of air is a parasitic loss to the engine thermodynamic cyclecausing degradation in efficiency.

Secondary airflows are metered via air-to-air seals that are typicallyplaced at interstage locations in the turbine engine. Relativevelocities are high, typically about 150 meters/second (about 500ft/sec) and up, and temperatures in the turbine section of the engine ofabout 425° C. (about 800° F.) and greater are typical. Since thebeginning of the gas turbine, labyrinth seals have traditionally beenused to seal these locations. Unfortunately, large thermal gradients,particularly during start up and shut down, result in considerableradial and axial excursions between the rotating and stationary parts ofthe seals. This makes it difficult to minimize operating clearances andso leakage through these seals, and the associated power loss, isusually significant.

It has been known for some time that if better seals were availableengine performance could be substantially improved. See, for example, J.Munson and G. Pecht. “Development of Film Riding Face Seals for a GasTurbine Engine.” STLE Tribology Transactions. V35 (1992). 1, 65-70. Thisreference shows that the use of just three advanced seals could reducedirect operating cost of a modem regional jet by almost 1%. Thissubstantial benefit was the result of reduced fuel consumption andreductions in chargeable maintenance while producing the same poweroutput at a lower turbine inlet temperature than an engine withconventional seals. In order to achieve these benefits it was necessaryto place advanced mechanical seals very near to the blade/vane gaps inthe high pressure turbine. Munson et al goes on to indicate that theselocations are among the most difficult to seal because of the speed,temperatures, large excursions, and the inability to keep parts flat dueto the large thermal gradients which characterize these locations.Munson et al provides a table of expected deflections and distortions atthe three advanced seal locations along with speeds, temperatures, anddifferential pressure range.

2. Related Art

Efforts to provide improved seals for use in gas turbines and otherapplications have led to the production of abradable coatings forlabyrinth seal stators and variations of labyrinth tooth geometry, andto the development of brush seals. These seals attempt to provide alabyrinth seal tooth with some compliance. The compliance allows theseals to track radial clearance excursions with only minimal wear of theseal. Leakage thus remains lower for a longer time relative to alabyrinth seal operating at the same location.

Over the past thirty years, several researchers tried to adaptmechanical face seals for use as advanced secondary air seals. Probablythe earliest large effort in this direction is that described by L.S.Dobeck in an article entitled “Development of Mainshaft Seals forAdvanced Air Breathing Propulsion Systems,” Pratt & Whitney Aircraft,NASA CR-121177 (1973). The focus of this effort was to modify theoil-cooled face seals already in use in engine bearing sumps to aconfiguration that did not require oil cooling. This program introducedthe film-riding or gas-lubricated face seal. Later efforts followed, forexample, see P. Lin-wander, “Development of Helicopter Engine Seals,”AVCO-Lycoming, NASA CR-134647 (1973) and “Self-Acting Seals forHelicopter Engines, AVCO-Lycoming. NASA CR-134940 (1975); M. O'Brien,“Development of a Short Length Self-Acting Seal,” AVCO-Lycoming. NASACR-135159 (1976); and the 1992 J. Munson and G. Pecht article notedabove. The work describes efforts to increase both the stiffness of thegas films and the demonstrated operating conditions. New lift featuressuch as spiral grooves, etc., are described and new materials such assilicon carbide are introduced to overcome temperature limitations ofcarbon graphite.

More recently, J. F. Gardner et al U.S. Pat. No. 5.769,604 describes adouble spiral groove hydrostatic-type seal. If one or both of the sealfaces should experience a conical distortion in operation, these spiralgrooves would tend to produce a moment on the seal faces in the oppositedirection. To take advantage of this righting moment the stationary orprimary seal ring has deliberately been made thin and flexible, theremainder of the seal follows typical face seal design practice. Theintent of this design is to allow the hydrostatic seal toself-compensate for expected in-service conical distortion and thuspotentially extend its useful operational envelope. The concept iscurrently under development.

The devices described in the aforementioned references describehydrodynamic face seal designs. These rely primarily on the relativerotation of the seal faces to generate the lift force that separates theseal faces. The conclusion from review of this work is that the thin gasfilms that characterize this type of seal allow almost no distortion ofthe seal faces. In other words, even minute distortions in the sealfaces must be prevented if adequate performance is to be achieved. Inapplications where this can be guaranteed successful applicationsresult. For example, hydrodynamic designs have come to dominate the gaspipeline and process industry applications where distortion can becontrolled, as described by P. E. Hesie and R. A. Peterson. “MechanicalDry Seal Applied to Pipeline (Natural Gas) Centrifugal Compressors,”ASME-ASLE 1984 Joint Lubr. Conf., Preprint 84-GT-3. Where this cannot beguaranteed, such as inside a gas turbine engine, success has provedelusive.

Hydrostatic seals provide an alternative to hydrodynamic face sealdesigns. These only need an applied differential pressure. Hydrostaticdesigns work best with thin gas films, on the order of 0.0001 inch, butthey can also operate with 10 times this film thickness. This increasesthe acceptable amount of distortion that the seal can tolerate withoutcontact between the relatively rotating seal faces.

Turnquist, Tseng et al describes the development of a large hydrostaticface seal for use in an aircraft gas turbine engine in “Analysis andFull Scale Testing of an Aspirating Face Seal With Improved FlowIsolation”, 34^(th) AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Jul.13-15. 1988, Cleveland, Ohio. The thick gas film allows the seal to copewith the expected distortion levels of the seal faces when operated inthe engine. Leakage through this type of hydrostatic seal is much higherthan that which would be expected from an equivalent diameterhydrodynamic face seal. However, it is pointed out that leakage is muchlower than that which can be obtained from any other potential sealtype, and is still expected to provide over 1% savings in enginespecific fuel consumption (SFC). During engine start and shutdown,differential pressure is sometimes insufficient to separate the sealfaces. To overcome this an “aspirating” labyrinth seal tooth has beenapplied in parallel with the seal. This allows the seal to remain “open”(non-contacting) until sufficient differential pressure is available tosupport the seal faces. This seal has undergone considerable rigdevelopment testing. It has met all of its test objectives. It is to beground tested in a gas turbine engine in the near future.

Although promising for some gas turbine applications, hydrostatic sealscannot be utilized when the surface speed is high, typically over 305meters/second (m/s) (1000 ft/s).

Allison Engine Company conducted an extensive literature survey of foilbearing capability prior to committing to the hybrid seal concept.Approximately 375 citations were reviewed covering the period from 1990to the present. Although no references to foil face seals were reported,the literature survey revealed the existence of an extensive foiljournal and thrust air bearing design, test, and manufacturing base. Thegreat advantage of the foil design over the fixed geometry designs isthe conforming nature of the foils. These have been shown to accommodatethermal and dynamic shaft and housing deflections. When used in journalbearing applications they have also demonstrated the ability to preventhalf-speed whirl indicating that they are capable of providing stableoperation.

SUMMARY OF THE INVENTION

This invention provides an improvement in a gas riding face seal formounting between mutually rotating structures. The improvement comprisesa flexible top plate that has a generally undulate face surface at leastwhen it is bearing an axial load, to generate a riding gas film, andsupport means for rendering the top plate compliant to out-of-flatdistortions and axial excursions between the rotating parts whilemaintaining a gas seal between such rotating structures. Optionally, thetop plate may have a generally undulate face surface, even when it isnot bearing an axial load. The support means may comprise a primaryspring system, for accommodating out-of-flat distortions, and asecondary spring system for accommodating axial excursions.

This invention also provides a face seal for mounting on a stationarystructure and facing an adjacent rotating structure, the face sealcomprising a flexible top plate having a generally undulate face surfaceand a support surface, a support spring system beneath the top platesupport surface for supporting the top plate over a stationary structureand rendering the top plate compliant to out-of-flat distortions andaxial excursions between the structures while maintaining a gas sealbetween the structures, and a ring seal secured to the top plate forestablishing a seal between the face seal and the stationary structure.

According to one aspect of this invention, the spring system maycomprise a support plate having two sides, a primary spring system onone side of the support plate, for accommodating axial excursions of thetop plate, and a secondary spring system on the other side of thesupport plate, between the top plate and the support plate, foraccommodating out of flatness distortion.

According to another aspect of this invention, the top plate maycomprise a plurality of overlapping sector plates on the outward surfaceto provide ramped sectors. Optionally, the sector plates may be hingedlyattached to the top plate. Alternatively, the top plate may comprise aplurality of sector plates on the face surface that are spaced from oneanother to define grooves between them.

According to another aspect of this invention, the seal furthercomprises a case for mounting sealingly onto the stationary structureand within which the top plate, the primary spring system, and thesecondary spring system are mounted, and wherein the ring sealestablishes a seal between the top plate and the case.

Optionally, the top plate may comprise a low-friction, low-wear surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a static vane and tworotors adjacent thereto in a gas turbine and of portions of two faceseals in accordance with this invention;

FIG. 2A is an exploded perspective view of a face seals according to oneembodiment of the present invention;

FIG. 2B is a cross-sectional view of the face seals of FIG. 2A, asassembled;

FIG. 3 is a schematic cross-sectional view of the top plate of the faceseals of FIGS. 2A and 2B; and

FIG. 4 illustrates an alternative embodiment of a top plate for a faceseals according to this invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

Gas film riding seals have found wide acceptance in pipeline and processindustry turbomachinery. However, such seals have not found utility ingas turbine prime movers.

The gas turbine operating environment requires that the seals operate atrelatively high surface velocities and higher temperatures than those ofall other applications, and flight-worthy gas turbines must meetadditional restrictions on the allowable size and weight of the seal.Furthermore, the structural portions of an aircraft engine arelightweight, which means that the seal cannot be completely isolatedfrom the distortion of the surrounding structure. It is this lastproblem which more than anything else has prevented the use offilm-riding bearings in gas turbine aircraft engines.

This invention provides a hydrodynamic foil face seal that is muchbetter than prior art devices at accommodating axial excursions,misalignments, out-of-flatness, conical distortion and circumferentialdistortion. Such a face seal comprises a top plate having a seal facethat has (at least when it is bearing an axial load) a generallyundulate (i.e., not flat, but stepped, sinusoidal, etc.) surface assensed rotationally, i.e., as sensed along a circular path on thesurface of the seal face. The top plate has a seal face (i.e., anoutward facing top surface) on one side, and a top plate support surfaceon the other. In addition, the top plate is flexible and the sealassembly is designed to accommodate axial misalignments, seal facedistortions, and axial excursions between the seal and the rotating partit faces. As a result of the improved tolerance of the foil face seal ofthis invention for such deviations, the invention finds use in gasturbines, including aircraft engines.

Typically, a seal according to this invention is mounted on a stationarystructure with the seal face disposed against, or adjacent to, anadjacent rotating structure. For ease of expression, the structure onwhich the seal is mounted will be referred to herein as the stationarystructure, and the structure that the seal faces will be referred to asthe rotating structure or “disc”.

The seal of this invention has demonstrated an ability to handle sealface distortion far in excess of any other gas film riding seal. It canoperate over a wide range of diameters and a wide range of pressuredifferentials and so can be used in a variety of locations within aturbine, e.g., about 30 to 46 cm (about 12 to 18 inches). A preliminaryengine design study produced anticipated operating conditions for theseal at three selected turbine rim locations. The first location listedin TABLE I (location 1V-1B) is between the turbine inlet vane and thefirst stage turbine rotor. The second two locations (1B-2V, 2V-2B) seala vane located between two turbine stages, as depicted schematically inFIG. 1 wherein foil seals 10, which do not include casings, are mountedin annular grooves in a turbine stage 100 to provide a dynamic sealrelative to rotating vanes 102 and 104. This is a very common turbinesealing application. The anticipated operating conditions for such adevice are presented in TABLE I.

TABLE I Anticipated Turbine Rim Operating Conditions Location 1V-1B1B-2V 2V-2B Temp.° C. 620 598 598 (Max. Source) (1148° F.) (1108° F.)(1108° F.) ΔP Max (atm)  1.76  1.959  3.796 (25.9 psid) (28.8 psid)(55.8 psid) ΔP Min (atm)  0.0013  0.0388  0.427 (0.02 psid) (0.57 psid)(6.28 psid) Conical  0.13  0.32  0.52 Distortion (degree) Circ. Out of 0.762  0.1016  0.2286 Flat. (mm) (0.003 in) (0.004 in) (0.009 in) Rel.Radial 1.016-2.286 2.032-3.302 0.508-2.032 Excursion (mm) (0.04-0.09 in)(0.08-0.13 in) (0.02-0.08 in) Rel. Axial 1.016-5.08 −0.762-+0.099−1.016-+4.826 Excursion (mm) (0.04-0.2 in) (−0.03-+0.13 in) (−0.04-+0.19in) Maximum 350.5 320 320 Speed (m/s) (1150 ft/s) (1050 ft/s) (1050ft/s)

The ΔP max and ΔP min indicate the range of the pressure differentialacross the seal surface. The “conical distortion” (or “conical out offlat”) indicates the angle formed by the top plate relative to a planeperpendicular to the axis of rotation. The “circular distortion” (or“circular out of flat”) indicates the amount by which an edge of theseal deviates from a plane at right angles to the axis of rotation ofthe rotors. Radial excursion refers to movement of the rotor relative tothe stationary vane in the direction along the axis of rotation. Axialexcursion refers to the movement of the axis of rotation relative to thestationary vane. Maximum speed refers to the maximum velocity of theouter circumference of the foil seal. The conical and circularout-of-flat distortions, and the very low differential pressures thatoccur at certain points in the operating cycle, are parameters that abearing in a gas turbine should accommodate, as well as high operatingtemperatures. Temperature is within the capability of existingmaterials.

This invention provides a face seal that can function within theforegoing parameters because the face comprises a flexible top plate orfoil that will flex to accommodate out-of-flatness distortions of theseal mating disc that is supported in a manner to permit such flexureand to accommodate much larger axial excursions between the rotating andstationary portions as well.

As mentioned above, the top plate of a seal according to this inventionhas an uneven seal face, at least when it is subjected to an axial load.It is the non-flat configuration of the seal face relative to the discthat allows the seal to generate the gas film between them at sufficientrotational speed. The non-flat surface can be inherent on the top platein its non-loaded configuration, or the top plate can have an initiallyflat face when at rest, but in such case, the seal must be configured toallow the top plate to deform into a non-flat configuration whensubjected to an axial load from the disc. This can be achieved, forexample, by providing a top plate with a normally flat face on a supportplate that provides an irregular support surface.

The seal comprises a support spring system (i.e., one or more springsand associated support structures) beneath the top plate. The supportspring system supports the top plate over the stationary structure andpermits it to conform to the various distortions, misalignments andexcursions described above. One such support system provides one or moresprings that support the top plate, but which permit it to flex inresponse to the out-of-flatness distortions, and one or more springs topermit the top plate to move axially along the axis of rotation of therotating part.

Once in operation, the gas film between the top plate and the discinhibits gas flow between the inner and outer regions of the seal, thuspermitting the maintenance of a pressure differential across the sealface. To prevent gas flow around the top plate, a seal must beestablished between the seal structure and the stationary member onwhich it is mounted. This is achieved by providing a seal ring aroundthe inner or outer circumference of the top plate, and between the faceseal and the stationary structure. Such an arrangement is illustrated inFIG. 1, wherein labyrinth ring seals are secured around the inner edgesof the top plates and bear against groove walls in the stationarystructure. Optionally, the seal assembly can be mounted in a case. Insuch case, a seal ring is positioned between the case and the top plate,and the case is mounted on the stationary member in a manner thatprevents gas flow between the case and the stationary member.

A first embodiment of a face seal according to this invention is shownin the exploded perspective view of FIG. 2A and in the cross-sectionalview of FIG. 2B. Referring now to both FIGS. 2A and 2B, seal 10 having adiameter of about 12 centimeters (cm) (4.7 inches) comprises an annularface seal case 12, a spring base plate 14, a support plate 16, springs18, a bump plate 20, a top plate 22 and a seal ring 24. Seal 10 alsocomprises two retaining rings, an inner retaining ring 26 and an outerretaining ring 28.

Face seal case 12 defines a flat face 12 a and two concentric flanges,including an inner flange 12 b having an interior diameter of about 4.1cm (1.6 inches) and a thickness of about 0.63 millimeters (mm) (0.025inch) and an outer flange 12 c. In the orientation shown in FIGS. 2A and2B, outer flange 12 c rises to a greater height than inner flange 12 brelative to face 12 a by about 2.54 mm (0.1 inch). Together, face 12 aand flanges 12 b and 12 c define an annular channel within which theother structures of this particular embodiment are disposed. Theexterior of case 12 is smooth and configured so that when mounted on astationary structure a physical seal is easily formed between thestationary structure and the case.

Spring base plate 14 is annular in configuration and rests directly onface 12 a within flanges 12 b and 12 c. The support spring system ofthis embodiment comprises a plurality of springs 18 (referred to hereinas “secondary springs”) mounted on spring base plate 14 to support asupport plate 16 in spaced relation to spring base plate 14. Springs 18are configured to bear the expected axial load and to accommodate theexpected axial excursions set forth in Table I above. The support springsystem also comprises the annular bump plate 20, which is mounted onsupport plate 16. Bump plate 20 carries a plurality of primary springs20 a mounted thereon. Primary springs 20 a can be mounted on theunderside of plate 20 and bear against supporting plate 16 or on the topside of plate 20 (as shown) and bear against top plate 22. In theillustrated embodiment, primary springs 20 a are in the form ofcorrugated metal sector-shaped coupons that are configured to occupyannular sectors of bump plate 20 and are each secured to plate 20 b atone edge thereof Springs 20 a are configured to bear the expected axialload and to accommodate out-of-flatness deviations indicated in Table I.For this purpose, the corrugations flatten somewhat in response topressure from top plate 22 and thus provide a spring-like cushioningeffect.

Annular top plate 22 is mounted on the bump plate 20 and seal ring 24 isdisposed therein. Seal ring 24 establishes a seal between top plate 22and case 12 and, in the illustrated embodiment, is configured to providea labyrinth seal at inner flange 12 b, but other types of seal may beused in its place, e.g., a bush seal, a metal o-ring, a segmented tiltpad seal (as shown, e.g., in the article by J. Munson, D. Grant and G.Agrawal “Foil Face Seal Development”, AIAA 2001-3483 presented at the37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Jul. 8-11, 2001,Salt Lake City. Utah). In addition, the seal may be formed about eitherthe inner or the outer circumference of the top plate and the casing.Several keys 30 are situated within case 12, and plates 14, 16, 20 and22 are all notched to mate with the keys 30 and thus maintain theirorientation within face seal case 12 at all times. Outer retaining ring28 retains plates 14, 16, 20 and 22 within face seal case 12.

As seen in FIG. 2B, support plate 16, bump plate 20 and top plate 22 areall configured to be situated between flanges 12 b and 12 c (i.e.,within case 12), and at an offset from the interior of flange 12 b. Sealring 24 is situated within the offset between inner flange 12 b and topplate 22. Inner retaining ring 26 holds seal ring 24 in place on faceseal case 12. Seal ring 24 defines a mounting flange 24 a and isconfigured to overlap supporting plate 16 and thus retain plate 16 inface seal case 12. The seal has an annular configuration that defines aninterior region 40 and an exterior region about the outer perimeter ofthe seal.

The structure of top plate 22 is illustrated in FIG. 3. Top plate 22 hasa flat bottom support surface 22 d that contacts bump plate 20. The topsurface (or “face”) of top plate 22, however, is stepped so that theheight of the face of top plate 22 relative to the bottom supportsurface. 22 d is generally uniform along any given radius, but itchanges in a rotational direction, e.g., as sensed in the rotationaldirection of arrow 22 c (FIG. 2A). The top surface of top plate 22 has asawtooth-like rotational surface profile in which the overall thicknessof the top plate repeatedly decreases quickly and then increasesgradually in the direction of arrow 22 c. One way this may be achievedis by securing a series of thin, flat, circularly arranged overlapping,sector-shaped pads or scales 22 b (FIG. 3) onto a thin, flat, annulartop base 22 a. The exposed surfaces of pads 22 b provide a surface fortop plate 22 that has a generally sawtooth-like configuration with anamplitude of about 0.1524 mm (0.006 inch) as sensed moving in a circularpattern about the center of the assembly, as suggested by FIG. 3. Thus,top plate 22 has a generally undulate face surface even when it is notbearing an axial load. In other embodiments, the top plate may have aflat face when not bearing a load, and there may be structures beneaththe top plate which, when the top plate bears an axial load, deforms thetop plate so that the top surface is no longer flat, thus enabling it toform a gas seal between it and an adjacent rotating surface.

Preferably, the top surface of top plate 22, i.e., the top surfaces ofscales 22 b, is coated to provide a high-temperature, low-frictionsurface, to permit contact with a rotating structure with a minimum ofwear on the structure and on scales 22 b. For example, the surface canbe coated with a polyimid, or a ceramic high-temperature material.Alternatively, the disc can be coated if the top plate is not;optionally, both facing surfaces are coated. The coating prevents wearon the parts while they are in contact before the gas film is formed.

In use, bearing 10 may be secured to a section of a gas turbine thatfaces another and rotates relative to it, e.g., by securing case 12 to aturbine inlet vane with the stepped face of top plate 22 facing therotor. When seal 10 is concentric with the axis of rotation and otherconditions described herein, such as speed of rotation and axial load,are met, a gap will develop between the face seal device and therotating part (i.e., the rotor), where a thin film of air will bemaintained between them. The thin film of air will protect the matingsurfaces from wear and will also establish a barrier to the transfer ofgases through the gap. As a result, a pressure differential can bemaintained between the interior and exterior of the annular face sealdevice, thus serving a necessary requirement for a bearing in a gasturbine. The primary springs 20 a enable the device to accommodateprimary deviations in the seal, e.g., misalignments, out-of-flatness,conical distortion and circumferential distortion, while the secondarysprings 18 enable the device to tolerate substantial axial movement. Theaxial movements accommodated by the primary springs and the secondarysprings may be expected to differ by up to two orders of magnitude andthe springs are chosen accordingly.

A prototype face seal-like the one shown in FIGS. 2A and 2B was testedin a test rig. The test seal had a diameter of about 12.7 cm (5 inches),and included a top plate that had eight pads, each having a thickness of0.524 mm (0.006 inch). The inner diameter of the top plate measuredabout 5.97 cm (2.35 inches) and the outer diameter measured about 10.77cm (4.24 inches) and had a circumference of about 33 cm (13 inches). Inthe test ring, the face seal was mounted on a non-rotating piston thatprovided axial load at typical operating levels, and air bearingslocated the piston shaft radially. A rotatable thrust runner waspositioned against the face and was rotated at 60,000 rpm, giving acircumferential velocity of about 335.3 meters/second (1100 feet persecond). Various levels of thrust load were applied against the face ofthe bearing and the bearing was spun up to surface speeds equivalent tothose shown in TABLE I and then allowed to coast down. Torque wasmeasured continuously during these tests. The speed at which lift-offoccurs, i.e., at which the film of air develops between top plate 22 andthe thrust runner, was documented by noting the precipitous change intorque that accompanied lift-off. A family of curves was generated byrepeating the same experiment with increased thrust load. These dataallow comparisons of film stiffness and load capacity to be made betweenseals of different design. High film stiffness and load capacity arepreferred. The prototypes that have been tested will all support anominal 10 psi bearing load. The protoyypes that have been tested willall support a nominal 10 psi bearing load.

A face seal according to FIGS. 2A and 2B was tested repeatedly forconical distortion of the top plate of 0.52° and 0.32°. For thispurpose, two simulator plates were manufactured to simulate the expectedconical distortion between the thrust runner and the face seal. Thedistortion was created in the top plate by installing the top plate andthe bump plate on the simulator plate, which was used instead of theflat support plate 16 in FIG. 2A. The face seal results were comparedwith flat face seal results for performance and load capacity. Testingwas done as described above using the standard flat backing plate beforethe conical tests and then after the testing was completed. There was nosignificant difference in torque results. These data show that a faceseal bearing device according to this invention will continue tofunction despite conical distortion. The outcomes of this analysis alsoindicate that the face seal of this invention has a measured loadcapacity higher than the 27.25 kg (60.0-pound) maximum test load thatwas applied. The data obtained from 0.52° and 0.32° conical distortiontests are compared with each other in TABLE II. There are very smalldifferences between the torque values for each applied load.

TABLE II Comparison of Measured Torque Values at Conical Distortions of0.52° and 0.32° Axial Load Torque at 0.52° Torque at 0.32° (lbs.)(in-lbs.) (in-lbs.) 10 0.35 0.31 20 0.47 0.40 30 0.64 0.61 40 0.84 0.75

Several more simulator plates were manufactured with circumferentialundulation, i.e., deviation from flatness, and were incorporated intoface seals as shown in FIGS. 2A and 2B. Testing was conducted asdescribed above with plates that had one wavelength per circumferenceand an “out-of-flatness” or wave amplitude of 0.2286 mm (0.009 inch) and0.1016 mm (0.004 inch), and one, one-and-one-half, and two waves percircumference. In all cases, the top plate and bump plate were installedon top of the undulate simulator, which caused the top plate to take onthe shape of the simulator plate.

The circumferential undulation test results for the one wavelength percircumference testing are compared with the standard flat test in TABLEIII. The thin-film portion of the seal successfully tracked all of thewavy plates. No contact was observed. Torque seemed to decrease slightlyas the number of waves increased, but this was a minor effect. The dataare set forth in Table III.

TABLE III Torque Values at Normal and 0.2286 mm (0.009 Inch) and 0.1016mm (0.004 Inch) Out of Flatness Torque (in-lbs) Cir. Torque (in-lbs)Cir. Out-of-Flatness Out-of-Flatness Torque From Axial Load 0.2286 mm0.1016 mm Normal (lbs.) (0.009 inch) (0.004 inch) Test (in-lbs.) 10 0.256 in-lbs. 0.218 in-lbs. 0.320 20 0.1755 0.148 0.432 30  0.277 0.2700.510 40  0.351 0.351 0.620 50  0.410 0.425 0.680

The data of Table III show that by making at least a portion of theseal, e.g., the top plate and the bump plate, the seal can be made totrack very large distortions of the rotating mating ring relative toprior art film riding face seal technology, e.g., distortion levels thatare more than two orders of magnitude greater than what conventionalface seal bearing can tolerate. The torque readings for thecircumferentially undulate tests are lower in both cases than for thestandard flat plates. This can be explained if the assumption is madethat more of the axial load is being carried by the peak areas of theundulation. The average film thickness therefore is increased and, sincetorque is inversely proportional to the third power of film thickness,one would expect a decrease in torque with an increase in average filmthickness. In general, reduced torque is welcomed because it indicatesthat greater loads can be sustained. However, as load (and torque)increase, the minimum film thickness at the peaks of the undulationsdecreases and eventually, asperities on the facing surfaces will contactand wear will result. Another disadvantage is that the leakage from theseal will also increase since this is also proportional to filmthickness.

The seals of this invention are believed to offer several advantagesover prior art face seals. For example, segmentation of the top platesurface by the use of pads allows the manufacture of seals of varioussizes because the number and orientation of pads being used can bevaried to suit the size of the top plate without influencing theperformance of any particular segment. For example, FIG. 4 illustrates atop plate 122 having two concentric arrangements of pads, an outer ringof pads 122 b, and an inner ring of pads 122 b′, on a top base 122 a.

FIG. 4 also illustrates a top plate 122 that comprises pads that do notoverlap, but which nonetheless yield a generally undulate seal facebecause the pads are situated to leave grooves 122 c between them.

Some advantages of this invention are that the seal is relativelylightweight since most of it is constructed of sheet metal; it isrelatively inexpensive since it is constructed primarily of readilyavailable sheet metal and it is not necessary to manufacture fine groovefeatures to generate lift since the pads distort to create lift on theirown; it should be relatively robust since there are no fine features toclog in service; and there are no limitations on increasing the size ofthe seal to diameters required for the turbine rim locations. Theability of the pads to rise from the top base allows the seal to respondlocally to local distortions in the air film due to changes in operatingconditions, foreign objects in the film, etc.

Advantages of this invention include the ability of the seal to trackcircumferential out of roundness; the ability to easily scale the designby simply adding or subtracting segments; the lack of need for finegeometry to generate lift using the tilt pad concept; and superiorrobustness of the seal since there would be no fine geometry featuresthat could wear or become clogged in service.

Although the invention has been described with reference to particulardisclosed embodiments, it will be understood by one of skill in the art,upon a reading and understanding of the foregoing disclosure, thatnumerous alterations and variations to the disclosed embodiments can bemade and are intended to fall within the scope and spirit of theinvention as defined in the appended claims.

1. In a gas riding annular face seal for mounting between a firststructure and an adjacent second structure so that the annular face sealseparates a first region from a second region, at least one of thestructures being rotatable relative to the other, the improvementcomprising: the face seal comprises a flexible annular top plate formounting on such first structure; the top plate having an undulate facesurface, at least when such structures are rotating relative to eachother, for generating a riding gas film between the top plate and suchsecond structure, the top plate (a) being sufficiently flexible to becompliant to out-of-flat distortions between the first and secondrelatively rotating structures, and (b) having a plurality of sectorplates on the face surface; a support spring system for rendering thetop plate compliant to out-of-flat distortions and axial excursionsbetween the rotating structures while maintaining the riding gas film;and a seal ring positioned between the top plate and the structure onwhich the face seal is mounted to establish a barrier to gas flow acrossthe face seal, whereby the face seal is able to maintain a pressuredifferential between the first and second regions.
 2. The face seal ofclaim 1 wherein the seal ring is secured to the top plate.
 3. The faceseal of claim 2 wherein the first structure is a stationary structureand the second structure is rotatable.
 4. The face seal of claim 1wherein the seal ring comprises a labyrinth seal.
 5. The face seal ofclaim 1 wherein the support spring system comprises a primary springsystem for accommodating out-of-flat distortions, and a secondary springsystem for accommodating axial excursions.
 6. The face seal of claim 5wherein the support spring system comprises a support plate having twosides, wherein the primary spring system is on one side of the supportplate, and the secondary spring system is on the other side of thesupport plate from the primary spring system.
 7. The face seal of claim6 wherein the primary spring system is between the top plate and thesupport plate.
 8. The face seal of claim 5 wherein the top platecomprises a low-friction, low-wear surface.
 9. The face seal of any oneof claims 1, 2, 4 or 5 wherein the plurality of sector plates on theface surface overlap each other to provide an undulate surface even whenneither of such first and second structures are rotating.
 10. The faceseal of claim 9 wherein the sector plates are hingedly attached to thetop plate.
 11. The face seal of claim 1 wherein the top plate is flatwhen at rest, and wherein the seal is configured to allow the top plateto deform into a non-flat configuration when such structures arerotating relative to each other.
 12. The face seal of any one of claims1, 2, 4 or 5 wherein the plurality of sector plates on the face surfaceare spaced from one another to define grooves between them to provide anundulate surface even when neither of such first and second structuresare rotating.
 13. The face seal of any one of claims 1, 2, 4 or 5wherein the seal is mounted on such first structure adjacent to suchsecond structure, and the first structure is a stationary structure andthe second structure is rotatable.
 14. The face seal of any one ofclaims 1, 2, 4 or 5 wherein the seal is mounted on such first structureadjacent to such second structure, and the first structure is rotatable.15. In a gas riding annular face seal for mounting between a firststructure and an adjacent second structure so that the annular face sealseparates a first region from a second region, at least one of thestructures being rotatable relative to the other, the improvementcomprising: the face seal comprises a flexible annular top plate formounting on such first structure, and a seal ring positioned to seal theface seal against gas flow across the face seal; the top plate having anundulate face surface, at least when such structures are rotatingrelative to each other, for generating a riding gas film between the topplate and such second structure, the top plate (a) being sufficientlyflexible to be compliant to out-of-flat distortions between the firstand second relatively rotating structures, and (b) having a plurality ofsector plates on the face surface; and a support spring system forrendering the top plate compliant to out-of-flat distortions and axialexcursions between the rotating structures while maintaining the ridinggas film, the support spring system comprising a primary spring systemfor accommodating out-of-flat distortions, and a secondary spring systemfor accommodating axial excursions; and a case which is dimensioned andconfigured for being sealingly mounted onto such first structure, andwithin which the top plate, the primary spring system, and the secondaryspring system are mounted, and wherein the seal ring establishes a sealbetween the top plate and the case, whereby the face seal is able tomaintain a pressure differential between the first and second regions.16. A method for maintaining a pressure differential across a gas ridingannular face seal mounted between a first structure and an adjacentsecond structure to separate a first region containing gas at a firstpressure from a second region containing gas at a second pressure whichis different from the first pressure, the face seal comprising (a) aflexible annular top plate mounted on a first structure which isadjacent to a second structure, at least one of the structures beingrotatable relative to the other; the top plate having an undulate facesurface, at least when the structures are rotating relative to eachother, for generating a riding gas film between the top plate and thesecond structure, the top plate (i) being sufficiently flexible to becompliant to out-of-flat distortions between the first and secondrelatively rotating structures, and (ii) having a plurality of sectorplates on the face surface; (c) a support spring system for renderingthe top plate compliant to out-of-flat distortions and axial excursionsbetween the rotating structures while maintaining the riding gas film;and (d) a seal ring positioned between the top plate and the structureon which the face seal is mounted to establish a baffler to gas flowacross the face seal, whereby the face seal is able to maintain apressure differential between the first and second regions, the methodcomprising: rotating at least one of the first and second structuresrelative to each other at a rate at least sufficient to establish andmaintain the riding gas film.
 17. The method of claim 16 comprisingmaintaining the first structure stationary and rotating the secondstructure.